The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device so as to increase process performance based on optimization of a mask layout in the lithography process of a semiconductor manufacturing method.
Transfer of a pattern to a resist is performed through the steps of irradiating coherent light to the so-called photomask, i.e., a glass plate having a device pattern of a light shield material formed on a glass substrate, and then projecting the diffracted light having passed through the photomask onto the resist in the same or reduced size by a projection lens. An optical image of the projected light causes a photochemical reaction in the resist to produce a difference in solubility with a developing solution between an irradiated area and a non-irradiated area. Such a difference provides a dissolution contrast in the developing step, thereby forming the device pattern in the resist. Therefore, the optical image projected onto the resist greatly affects the accuracy of a final resist pattern.
The transfer of the mask pattern to the resist is performed by utilizing optical characteristics of the projected light. As a consequence, an unintended pattern is possibly transferred to the resist due to light interference depending on the mask pattern. That phenomenon appears when the pattern size is very fine. More specifically, when two adjacent patterns on a mask are arranged near or under a resolution limit, a resolution capability is not sufficient to separate the two patterns from each other. Therefore, the diffracted lights having passed through the two patterns interfere with each other so that the patterns are joined together on the resist. Further, the interference of projected and diffracted light similarly appears and an optical image is moderately curved in or near an area of a wiring pattern where the pattern bends at an internal angle of not larger than 180° (or at an external angle of not smaller than 180°) or where the pattern has a different line width. Accordingly, rectangularity deteriorates in an inwardly angled area of the resist pattern. Such a deterioration of the transfer accuracy may cause serious defects, e.g., an increase of parasitic resistance and capacity in wiring and gate portions, a decrease of driving performance of transistors, and an increase of contact resistance due to a contact failure at a via for connection to an upper wiring layer.
In order to improve fidelity in transfer of a mask pattern, the mask pattern has hitherto been corrected by a method of estimating expected light interference and subtracting, on the mask pattern, the displacement of a transferred optical image caused by the interference in advance. That process is called OPC (Optical Proximity Effect Correction).
FIGS. lA through 1D illustrate several methods for reflecting, on the mask pattern, the displacement of an optical image caused by the interference in advance. A correction pattern having a specific shape is added for the purpose of correction in examples shown in FIGS. lA-1D. The examples include the case of applying a mask bias to a relevant area (as indicated by an original mask pattern M100 and a bias pattern MlOl), the case of preventing contraction at ends (as indicated by an original mask pattern M102 and an OPC correction pattern M103), and the case of improving rectangularity of angled corners (as indicated by an original mask pattern M104 and an OPC correction pattern M105). Another general method arranges a non-resolvable assist pattern S110 (as indicated by an original mask pattern M106 and OPC correction patterns M107, M108) to adjust periodicity in a pattern array and to provide more advantageous conditions from the viewpoint of process.
Still another method is further proposed which forms a slit having a non-transferable width at an inwardly angled portion of a mask pattern for wired lines including angled corners to suppress the light interference at the inwardly angled portion so that the resist pattern is prevented from unintentionally curving in the inwardly angled portion (see, e.g., Patent Reference 1; Japanese Unexamined Patent Application Publication No. 62-141558 (FIG. 1)).
In a mask pattern for wired lines including angled corners with a local difference in line width, transfer accuracy deteriorates near an inwardly angled portion of the mask pattern, and unintentional curving of the resist pattern appears at the inwardly angled portion. Therefore, such an inwardly angled portion is a target area for which the correction is to be made on the mask (as indicated by images of a mask pattern M109 and a resist pattern R109 in FIG. 2A) . According to the known method, the OPC is performed by forming the OPC correction pattern M107 shown in FIG. 1D. Further, in case where the correction method proposed in the above-cited Patent Reference 1 is used (as indicated by a mask pattern M110 in FIG. 2B), the transferred shape of a resist is improved as indicated by a resist pattern R110 in FIG. 2B, and the resist pattern curves more sharply in the inwardly angled portion.
However, in case where the mask pattern is an ultra-fine pattern having a scale so small as not larger than about ½ of the wavelength of exposure light used in an optical stepper, correction accuracy is very difficult to standardize. For this reason, a sufficient level of the accuracy cannot be obtained with the OPC correction pattern M107 that is used in the known method and shown in FIG. 1D. The mask pattern M110 shown in FIG. 2B is expected to provide a higher correction effect than the OPC correction pattern M107 shown in FIG. 1D. In the case employing high-resolution exposure conditions to resolve the ultra-fine pattern, however, a portion of very low light intensity appears near the inwardly angled portion due to excessive interference in the slit position, and the line width is extremely narrowed, thus leading to a line break (see a mask pattern M111 and a resist pattern R111 in FIG. 2C).
Even when a slit is formed in width at a level not affecting resolution, the slit includes an area where satisfactory transfer accuracy is not obtained, and an area where excessive interference promotes degradation of a transferred image. Therefore, the formation of the slit is very difficult in practicing the transfer of ultra-fine patterns.
Thus, the known methods of forming fine patterns have problems given below.
First, the accuracy of pattern transfer to a resist significantly deteriorates in an angled portion of a mask pattern for wired lines including angled corners with a local difference in line width. The reason is that diffracted lights interfere with each other in the angled portion, and consequently, the light contrast of a transferred optical image reduces.
Secondly, the known mask correction based on the OPC requires a lot of time and labor for calculating an optimum OPC correction amount. The reason is that an OPC correction pattern formed in the angled portion is a small 2D (two-dimensional) rectangular pattern, but a large number of combinations of parameters must be studied because a total of three parameters exist, i.e., length and width in addition to a position where the pattern is arranged.
Thirdly, when the mask pattern is an ultra-fine pattern having a scale so small as not larger than about ½ of the wavelength of exposure light used in an optical stepper, a sufficient level of transfer accuracy cannot be obtained with an OPC correction pattern of the known type added to a main pattern. The reason is that the ultra-small scale requires a very high level of correction accuracy. Further, since dimensions of the added OPC correction pattern are also much smaller than those of the main pattern, it is difficult to form an excellent correction pattern on the photomask, and consequently, a sufficient correction effect cannot be stably obtained.
Fourthly, in some cases, an ultra-fine line pattern has a scale so small as not larger than about ½ of the wavelength of exposure light used in an optical stepper and includes an angled portion. In those cases, even when a slit not resolvable on the photomask is formed so as to increase the transfer accuracy near the angled portion, an area having a very low light contrast appears near the angled portion, and a transferred pattern is often broken. The reason is that the diffracted lights having passed through the photomask tend to easily interfere with each other because the optical stepper used for resolving the ultra-fine pattern has very high performance. Therefore, even a slit having a width as small as not resolvable may rather lead to deterioration of the transfer accuracy in practice depending on the pattern dimensions.
Fifthly, even when a slit is formed in width at a level not affecting resolution, the slit includes an area where satisfactory transfer accuracy is not obtained, and an area where excessive interference promotes degradation of a transferred image. As a consequence, the formation of the slit is very difficult in practicing the transfer of ultra-fine patterns. The reason is that because the optical stepper used for resolving the ultra-fine pattern has very high performance, the optical image near the slit is adversely affected even with the slit having a very small width unless the slit width is optimized.